The invention relates to a hot runner nozzle according to the generic term of claim 1.
Hot runner nozzles are generally known. They are used in injection molds for feeding a flowable plastic material at a predefinable temperature under high pressure to a separable mold block (cavity). In order that the usually hot compound would not cool down prematurely within the nozzle, an electic heater is usually provided which concentrically encloses a material pipe and a flow channel formed therein, respectively and which is to hold the liquid plastic material at a constant temperature down to the nozzle tip if possible. A thermal separation between a hot manifold and the less warm mold ensures that the nozzlexe2x80x94especially in the zone of the nozzle tipxe2x80x94will not freeze and, at the same time, that the mold (or cavity) will not be heated up. A temperature sensor is normally used for temperature control.
The material pipe and the heater are often designed as separate components, the heater together with the temperature sensor being integrable in a jacket which may be peripherally attached onto the nozzle body. As disclosed e.g. in DE-U1-295 07 848, the jacket is a rigid structure which receives an electrically activated helix-shaped heating conductor and which may be fastened in an axial direction onto the nozzle body exterior in axial direction by means of holders or clamping elements. Alternatively flexible heating strips or mats are used which are fixed on the nozzle body (see e g. EP-B1-0 028 153 or WO 97/03540).
EP-B1-0 724 943 describes a hot runner nozzle comprising a heater designed as a preassembled heating unit with a cast body to be concentrically attached onto a material pipe. The heater is provided with a coiled filament centered by means of a perforated metal sheet, which filament forms a solid block together with the brass cast which block is then provided with a concentric bore to receive the material pipe. The manufacture of such heating systems is relatively complicated and expensive, what with special problems arising when a defective heater has to be replaced. Moreover, it is always necessary to replace the complete heating unit because the normally damaged filament is included in a cast. Consequenty, purchase and spare parts costs are high.
Another disadvantage of conventional heating devices is that the heater which concentrically encloses the material pipe takes relatively much room so that the nozzles cannot be arranged side by side as closely as perhaps desired. But many applications require that the spaces between the cavities are as small as possible so that separate cavities might be filled simultaneously or more complicated components might be filled by several shots at short distances.
To overcome this disadvantage, DE-U1-296 10 268 suggests to arrange the nozzle channel and the heater side by side in a common casing. An altogether flat, T-shaped casing comprises a base piece and a shaft piece whose free end winds up in a nozzle tip. Parallel to the straight nozzle runner, the casing is provided with a bore for receiving the heating unit designed as a rod-shaped heater that almost reaches to the nozzle tip. A problem of this arrangement is, however, that the complete casing consists of a uniform materialxe2x80x94normally tool steelxe2x80x94which has a negative effect on the heat distribution. Consequently the heating elements are often susceptible to trouble because the power, owing to the poor heat conductivity, is transferred to the casing to a limited extent only. This will result in frequent failures of the hot runner nozzle.
It is an object of the invention to overcome these and other disadvantages of the prior art and to create a hot runner nozzle having uniform heat-transition and temperature distribution curves and requiring little space when incorporated in a mold. Another object is the realization of a structure that can be manufactured and mounted economically and that guarantees reliable operation. Also aimed at is a nozzle layout with an arrangement of closely packed hot runner nozzles adapted to be produced at low cost and to be mounted quickly, reliable functioning also being very important.
In a hot runner nozzle for an injection mold with a material pipe made of a high-strength material, which may be mounted on a mold or manifold and is provided inside with at least one flow duct for a melt and at its lower end with a nozzle tip, and which comprises a heater for the melt to be attached onto the periphery of the material pipe, the invention provides that the heater is designed as a solid block made of a highly heat-conductive material and comprises a first receiving channel for the material pipe and at least one other receiving channel for a linear heating element, which other channel is substantially parallel to the first receiving channel. This extremely simple and cheaply realizable heating assembly ensures a surprisingly uniform and homogeneous temperature distribution in the material pipe down to the nozzle tip. The temperature set is maintained over the complete nozzle length, which fact will result in good final products.
This effect is also furthered if, in addition to the first receiving channel, at least two other receiving channels for heating elements are provided on one side or on both sides. Thus the necessary heating power may be easily distributed to several heating elements which may have smaller dimensions. This will have a positive effect on the service life of the heating system. The hot runner nozzles will always function reliably.
Moreover, the parallel arrangement of the receiving channels inside the heat-transferring block at a right angle to the axial direction requires little space so that several hot runner nozzles may be arranged closely side by side. Because of the small distances between the nozzles, such a row of nozzles may easily feed several cavities or several gates; extremely small distances between the cavities or gates, respectively, may be selected at least in a direction at a right angle to the axial direction.
Within the solid block the receiving channels may be aligned in one plane or be staggered radially and/or in relation to a longitudinal direction of the nozzle in order to create, for example, enough space for a temperature sensor. The spaces between the nozzles will not be increased by this arrangement.
Another essential advantage of the hot runner nozzle according to the invention is that the material pipe and the heating element, depending on the respective operating conditions, can be manufactured separately and from different materials, which will have a positive effect on the manufacturing costs. The heating elements may be inserted separately into the highly heat-conductive heater and may be replaced as required without requiring special dismantling work. The heating consists rather of a flat heater with quickly replaceable heating elements. It lends itself to manufacture by a few operations and may be attached to the pressure-tight material pipe non-positively or positively.
In order that the diameters of the heating elements and of the material pipes be variable, the cross-section of the solid block is smaller in the zone of the first receiving channel than in the zone of the other receiving channel(s), i.e. the solid block may be wider in the zone of the heating elements so that these may have larger dimensions. At the same time, the narrow spaces between the nozzle tips of neighboring hot runner nozzles will not increase so that small distances between cavities may still be realized.
The heating element is preferably an electric heating cartridge suitable to be connected by means of terminals to a control circuit known per se. However, the heating element may also consist of a pipe segment through which a heating medium is passed and which may either be inserted into the solid block or be formed thereby.
The material pipe is of substantially circular cross-section and may be inserted non-positively or positively into the first receiving channel of the solid block. Consequently, that channel may be designed as a bore which is easily made.
In order to be able to sense the temperature generated by the heater, the solid block is provided with at least one additional receiving channel for a temperature sensor.
Another important feature of the invention is that the solid block be fixed in relation to the material pipe. Thus it is avoided that the heater would be displaced or would inadvertently slip off the material pipe. For this purpose, the latter is preferably provided at its end with a peripheral groove into which a clamping element, e.g. a spring washer, is positively inserted. The outer diameter of the spring washer is greater than the outer diameter of the material pipe so that one end of the solid block rests on the spring washer and cannot slip off. Alternatively the spring washer may engage into the solid block non-positively or positively.
Still another aspect of the invention provides that a connecting piece of the material pipe has several shaft pieces arranged comb-like, each shaft piece being equipped with a solid block heater. Thus the individual nozzles form a nozzle battery with extremely small nozzle distances in a longitudinal direction. As each individual nozzle has its own heating, all nozzle channels may be individually supplied with the heating energy required.
Optimum heat transition is achieved if solid blocks or at least sections thereof are designed to be integral, each shaft piece being provided with a separate receiving channel and at least one other receiving channel for a heating element.
For their control, the heating elements are associated to separate control circuits each. However, it is also possible to assign groups of several heating elements to a common control circuit, whereby expenditures for control equipment will be reduced accordingly.
With a multiple-nozzle arrangement, the individual nozzles are arranged in close packing side by side, their side surfaces contacting each other, which will also result in extremely small distances between the cavities.